It looks like you gave the full dose, good for you for being brave! 279 is certainly a very safe number. I know you're apprehensive and worried because you have to leave but it looks like he's been okay today.
I noticed you asked if you should 'run out and get MC' - one thing that may help you feel more confident about shooting lower numbers - and sooner or later you're going to have much lower numbers to shoot - please stock up on a variety of medium and high carb wet foods. You'll want to have those on hand in the event of much lower numbers - such as in the 40's or 50's - just imagine it's late at night and you test and find him at 40 or even lower? You'll want to be sure to have those cans of food handy and marked as to the carb content of each one.
You did very well today! Just keep testing and let's see if we can get him down a bit lower